It is well known that cholesteric liquid crystal materials have a helical structure. With the cholesteric liquid crystal in a planar texture, the helical axis of the material is perpendicular to the surface of a liquid crystal cell. In the planar texture, the liquid crystal reflects a color light at the wavelength .lambda.=nP, where "n" is the average refractive index and "P" is the pitch length of the material. The width of the reflection band, or how much of the color spectrum is reflected, is .DELTA..lambda.=.DELTA.nP, where .DELTA.n is the birefringence of the material. Light incident upon a cell employing cholesteric liquid crystal material that is in a planar texture reflects circularly polarized light having the same twist sense as the liquid crystal material. In other words, right-handed cholesteric liquid crystal material reflects right circularly polarized light of the proper wavelength, but transmits left circularly polarized light.
In order to reflect both right and left circularly polarized light of a desired wavelength, two layers of cholesteric liquid crystal must be used. The first layer, which has a right twist sense, reflects right circularly polarized light and the second layer, which has a left twist sense, reflects left circularly polarized light. This reflection of the incident light is independent of the light polarization state, and as such the light in the reflection band is totally reflected.
Cholesteric liquid crystal material may be provided with a positive dielectric anisotropy. When a sufficiently high electric field is applied to such a material, the material is switched to a homeotropic texture and the material becomes transparent. When the high electric field is turned off, the liquid crystal material relaxes back to the planar texture and becomes reflective. Unfortunately, the relaxation time, that is the time it takes for the liquid crystal material to reconfigure from a homeotropic texture to a planar texture, is about 10 seconds. In many applications, a relaxation time of 10 seconds is much too long. One example of where a liquid crystal shutter may be used is in welding goggles. When welding, a welder needs to block or reflect any bright light that might otherwise damage his or her eyes. Goggles employing the above liquid crystal material would be effective in reflecting the harmful light, but would be inconvenient for use by the welder due to the long relaxation time.
Based upon the foregoing it is evident that there is a need in the art for a liquid crystal shutter that can transition between transparent and reflective states faster than 10 seconds. Furthermore, there is a need for such a liquid crystal shutter and an electric field driving waveform that minimizes the relaxation or transition time between transparent and reflective states.